Peripheral artery disease (PAD) is caused by atherosclerosis of the peripheral arteries, most commonly in the lower extremities, and is nearly as prevalent as coronary artery disease (CAD), with 8-12 million individuals affected in the US. PAD presents as either intermittent claudication (IC, pain with exertion that is relieved with rest or critical limb ischemia (CLI, pain at rest with or without tissue necrosis or gangrene). Less common than IC, CLI carries a substantially higher morbidity and mortality; CLI patients have a risk of major amputation or death that approaches 40% in one year. Evidence suggests that genetic differences play a role in the susceptibility to PAD, as inbred mouse strains have dramatically different responses to hind limb ischemia (HLI), a model of PAD. In C57BL/6 (BL6) mice, limb perfusion recovers without tissue loss, whereas BALB/c mice display poor recovery of limb perfusion and significant tissue necrosis, analogous to clinical CLI. In a screen for genes regulating limb survival in the mouse HLI model, a highly significant quantitative trait locus (Lsq 1) was identified. Lsq-1 contains the gene for Bcl-2-associated athanogene-3 (Bag3), which is required for skeletal myofiber survival and regeneration. Preliminary studies demonstrate that a single BAG3 polymorphism results in dramatic phenotypic differences in hypoxic skeletal muscle cells in vitro and in the mouse HLI model in vivo. Expression of the parental BALB/c variant, BAG3Met81, leads to skeletal myofiber atrophy and limb necrosis in vivo. In contrast, the BL6 variant, BAG3Ile81, completely rescues these defects with increases in myofiber size and vascular density in treated muscle. The central hypothesis of this proposal is that BAG3 variants are responsible for muscle survival and tissue loss with ischemia. To test this hypothesis, the Specific Aims of this proposal are to: 1) Determine the effects of BAG3 gain of function on skeletal muscle tissue necrosis and perfusion following limb ischemia in vivo; 2) Determine the cellular origin of BAG3's vascular effects in ischemia; and 3) Determine whether the protective role of BAG3 in ischemia is due to effects on autophagy. Although progress has been made in elucidating the contribution of genetic factors to PAD, identifying factors that modulate patients' susceptibility to CLI will be critical to understanding disease pathogenesis and in developing approaches to promote limb salvage for CLI and other ischemic diseases that currently lack effective treatments.